Studies revolving around avalanche ice dynamics show that melting and freezing of ice (and in essence ice-cream) would result in crystalline structures that are unfavorable to human taste (read "3D-characterization of three-phase systems using X-ray tomography: tracking the microstructural evolution in ice cream" paper published in a journal called "Soft Matter" July '12 issue). Now this is where we begin to reach a little bit, tapping on the glass by one person won't really have much of an affect on the icecream, but repeating the process by several people over a longer period of time will result in heat formation that will lead to melting and freezing on the surface of the icecream.
Second point. Dr. Masaru Emoto has researched the affect of sound and the environment of water as it freezes and the affect of said factors on crystal formation; a bad noisy environment created a hectic structure in the crystallized form of water, and a healthier quieter environment (also prayer or soft classical music) resulted in the best formation of water crystals that were beautifully arranged and produced the "best" quality of water. Icecream is 60% water on average (15 Hydration Facts for Athletes - By Nancy Clark) so disturbing the surroundings of water by tapping on the glass casing may in actual fact lead to an less optimal tasting ice cream.

tl;dr - Treat icecream nicely, it'll taste better.

and of course if you took this seriously, then may I suggest a little bit of icecream for your brain as it overworks itself, just to cool it down a little